Crack-tip plasticity and intrinsic toughening in nano-sized brittle amorphous carbon

Abstract

Most monolithic brittle materials are vulnerable to the failure by cracks because of a lack of intrinsic toughening mechanisms, such as the plasticity in the vicinity of the crack front. As a result, most of the efforts to mitigate the sudden failure of brittle ceramics have been focused on developing the extrinsic toughening mechanisms that hinder crack propagation behind the tip, such as the fiber bridging. In this work, we experimentally demonstrate that the intrinsic toughening arises even in the brittle monolithic ceramic material such as diamond-like carbon (DLC) when its external dimension reduces down to sub-micron scales. This unique phenomenon owes its origin to the decrease of the crack driving force in the small samples, which in turn enables them to bear high enough stresses to activate the local atomic plasticity. Through nanomechanical tensile and bending experiments, electron energy loss spectroscopy analysis, and finite element method for stress distribution calculation, we confirmed that the local atomic plasticity associated with sp 3 to sp 2 rehybridization is responsible for the intrinsic toughening.

Friction Control by Deformation Mode in Nanopatterned Amorphous Carbon

“…However, in light of the recent achievements in the field of nanomechanics that the extrinsic sizes and shapes of the materials also affect their mechanical behavior, 11−13 when their geometric dimensions decrease down to nanometers in scale. 11 It owes its origin to the abated crack driving force due to the volumetric reduction that escalates the fracture strength over the plasticity threshold. Because the mode I fracture is driven by the tensile stress while the shear drives the plasticity, the amount of the plastic deformation can be maximized by properly adjusting the boundary conditions, such as the sample geometries and loading directions, so that the shear components in the stress field prevail.…”

“…Because the mode I fracture is driven by the tensile stress while the shear drives the plasticity, the amount of the plastic deformation can be maximized by properly adjusting the boundary conditions, such as the sample geometries and loading directions, so that the shear components in the stress field prevail. 11 In other words, we can build the surface nanopatterns with varying geometric morphologies and dimensions to induce a different level of plasticity, as schematically presented in Figure 1(B). For example, in the pattern with a high aspect ratio, as shown in the top illustration of Figure 1(B), it is relatively hard to produce shear stress, and the constituent material, e.g., amorphous carbon in this work, deforms dominantly in an elastic manner, causing high elastic restoring forces until it loses the contact upon fracture.…”

“…In this case, whenever mechanical properties need to be changed, we must replace the materials themselves. However, in light of the recent achievements in the field of nanomechanics that the extrinsic sizes and shapes of the materials also affect their mechanical behavior, − we offer a new way to control the tribological behavior of nanopatterned surfaces by means of the topological alteration and resultant change in the deformation modes in the same material.…”

“…It has been reported that the plasticity can emerge even in the brittle materials, such as ceramics or carbon materials, when their geometric dimensions decrease down to nanometers in scale . It owes its origin to the abated crack driving force due to the volumetric reduction that escalates the fracture strength over the plasticity threshold.…”

“…It owes its origin to the abated crack driving force due to the volumetric reduction that escalates the fracture strength over the plasticity threshold. Because the mode I fracture is driven by the tensile stress while the shear drives the plasticity, the amount of the plastic deformation can be maximized by properly adjusting the boundary conditions, such as the sample geometries and loading directions, so that the shear components in the stress field prevail . In other words, we can build the surface nanopatterns with varying geometric morphologies and dimensions to induce a different level of plasticity, as schematically presented in Figure (B).…”